Tungsten(VI) fluoride, also known as tungsten hexafluoride, is an inorganic compound with the formula . It is a toxic, corrosive, colorless gas, with a density of about (roughly 11 times heavier than air). It is the densest known gas under standard ambient temperature and pressure (298 K, 1 atm) and the only well-characterized gas under these conditions that contains a transition metal. is commonly used by the semiconductor industry to form tungsten films, through the process of chemical vapor deposition. This layer is used in a low-resistivity metallic "interconnect". It is one of seventeen known binary hexafluorides.
Properties
The molecule is octahedral with the symmetry point group of O<sub>h</sub>. The W–F bond distances are . Between , tungsten hexafluoride condenses into a colorless liquid having the density of at . The vapor pressure of between can be described by the equation
:,
where the P = vapor pressure (bar), T = temperature (°C).
History and synthesis
Tungsten hexafluoride was first obtained by conversion of tungsten hexachloride with hydrogen fluoride by Otto Ruff and Fritz Eisner in 1905:
:
The compound is now commonly produced by the exothermic reaction of fluorine gas with tungsten powder at a temperature between :
:
The gaseous product is separated from Tungsten oxytetrafluoride|, a common impurity, by distillation. In a variation on the direct fluorination, the metal is placed in a heated reactor, slightly pressurized to , with a constant flow of infused with a small amount of fluorine gas.
The fluorine gas in the above method can be substituted by ClF, Chlorine trifluoride|, or Bromine trifluoride|. An alternative procedure for producing tungsten fluoride is to treat tungsten trioxide () with HF, , or Sulfur tetrafluoride|. And besides HF, other fluorinating agents can also be used to convert tungsten hexachloride in a way similar to Ruff and Eisner's original method:
: or
:
==Reactions==<!--needs an expert on the CVD part-->
On contact with water, tungsten hexafluoride gives hydrogen fluoride (HF) and tungsten oxyfluorides, eventually forming tungsten trioxide:
forms a variety of 1:1 and 1:2 adducts with Lewis bases, examples being .
Applications in the semiconductor industry
The dominant application of tungsten fluoride is in the semiconductor industry, where it is widely used for depositing tungsten metal in a chemical vapor deposition (CVD) process. The expansion of the industry in the 1980s and 1990s resulted in an increase in consumption, which remains at around 200 tonnes per year worldwide. Tungsten metal is attractive because of its relatively high thermal and chemical stability, as well as low resistivity () and very low electromigration. is favored over related compounds, such as Tungsten hexachloride| or Tungsten hexabromide|, because of its higher vapor pressure resulting in higher deposition rates. Since 1967, two deposition routes have been developed and employed: thermal decomposition and hydrogen reduction. The required gas purity is rather high and varies between 99.98% and 99.9995% depending on the application.
Hydrogen
The deposition process occurs at temperatures between 300 and 800 °C and results in formation of hydrogen fluoride vapors:
:
The crystallinity of the produced tungsten layers can be controlled by altering the /hydrogen| ratio and the substrate temperature: low ratios and temperatures result in (100)-oriented tungsten crystallites, whereas higher values favor the (111) orientation. Formation of HF is a drawback, as HF vapor is very aggressive and etches away most materials. Also, the deposited tungsten shows poor adhesion to the silicon dioxide which is the main passivation material in semiconductor electronics. Therefore, silicon dioxide| has to be covered with an extra buffer layer prior to the tungsten deposition. On the other hand, etching by HF may be beneficial to remove unwanted impurity layers.
Safety
Tungsten hexafluoride is an extremely corrosive compound that attacks any tissue. Because of the formation of hydrofluoric acid upon reaction of with humidity, storage vessels have Teflon gaskets.